FIG. 1 illustrates an aircraft 3 having tail-mounted gas turbine engines 6. The engines 6 each drive a fore propulsor 9F and an aft propulsor 9A which rotate in opposite directions about an axis.
FIG. 2 illustrates in greater detail the engine-propulsor system of FIG. 1. To the left is a gas turbine engine 15 such as the F404 type manufactured by the assignee of the present application. For purposes of the present invention, the gas turbine engine 15 can be considered as a gas generator which generates a high energy gas stream 33 and supplies the gas stream 33 to a propulsor stage 36.
The propulsor stage 36 extracts energy from the gas stream 33 directly, by means of low-speed counterrotating turbine blade sets. (This is in distinction to the common approach of using a high-speed turbine whose speed is reduced in route to a propulsor by a reduction gearbox.) A first set of blades 39 extracts energy from the gas stream 33 and spins the forward propulsor 9F. A second set of blades 42 spins the aft propulsor 9A, but opposite in direction to fore propulsor 9F. Bearings 47 support the blade sets and propulsors and allow this counterrotation.
A pitch-change mechanism 52 for changing the pitch of the propulsors 9A and 9F is shown schematically. It is desirable to modulate the pitch-change mechanism 52 so that the propulsor speed is proper under the prevailing operating conditions of the aircraft.
Various monitors are located in the engine 15 including sensors 17, 21 which provide signals representative of gas pressure (P2, P46) and sensor 19 which provides signals representative of inlet air temperature. Signal P2 (inlet air pressure) and signal P46 (air pressure exiting engine 15) are used to develop engine pressure ratio (EPR). While EPR is known to be the ratio P46/P2, it will be appreciated that for a constant P2, EPR can be obtained directly from a measurement of P46. Rotor speed is also sensed by a monitor 23 and provided as a control signal from engine 15. These sensors and others not shown are well known in the engine art. One control system for an engine such as engine 15 is described in U.S. Pat. No. 4,242,864, the disclosure of which is hereby incorporated by reference.
Furthermore, it is desirable to provide sufficient energy in the gas stream 33 to effect rotation of the propulsors 9A and 9F at a speed and resultant pitch angle so as to permit such proper operation, or, more specifically, to meet a pilot's demand for engine thrust. An example of aircraft propulsor control for a propulsor driven by a gas turbine engine is given in commonly assigned U.S. patent application Ser. No. 737,972 filed May 28, 1985, the disclosure of which is hereby incorporated by reference.